OK, you have a Mac behaving in an illogical manner. Could you tell us a little more about your Mac please, and the OS version you are running?
Then I would advise following the "recipe" below to try to get a known stable starting point:
First, check that your processor is running full speed. Go to Preferences, Energy Saver, Options and look at the drop box down near the bottom called Processor Performance. If it is not set to "Highest", set it to that right away. This maximizes performance, but for notebooks, it may run down the battery faster.
Next, download OnyX and run the complete set of clean up and maintenance scripts and then try again. Get OnyX at:
Finally, you may wish to check that you have enough free space on your hard drive. Highlight the Macintosh HD icon on your desktop, CTL-click it and select Get Info from the resulting menu. Make sure you have a reasonable amount of space left. If not, a little spring cleaning may be in order.
There are two excellent apps for showing where all of your hard disk space has gone, Disk Inventory X and WhatSize. Get them at:
Disk Inventory X: http://www.derlien.com
Both do a great job at letting you zero in on your largest disk space consumers, so that you can hunt down any rogue files.
Do the OnyX thing first, then restart and try out one or both of the above tools. By the way, all three are freeware!